H. Pfeifer

N.m.r. self-diffusion studies in zeolite science and technology

Abstract A review is given on the uses of nuclear magnetic resonance (n.m.r.) spectroscopy to investigate molecular migration in zeolitic adsorbate-adsorbent systems. The main parameters determining the transport properties of these systems are (a) the coefficients of intracrystalline and long-range self-diffusion, and (b) the molecular intercrystalline exchange rates. It is shown that these quantities may be determined directly by the n.m.r. pulsed field gradient technique in combination with the n.m.r. tracer desorption technique, thus allowing a complete description of the processes of molecular transport (‘diffusion analysis’). The variety of conditions to which molecular migration is subjected in the interior of the different types of zeolites gives rise to characteristic concentration dependences of intracrystalline self-diffusion. The n.m.r. studies yield at least five different patterns of concentration dependence. Being sensitive to the resonating nuclei (generally protons) of the adsorbed molecules, n.m.r. spectroscopy provides favourable conditions for self-diffusion measurements in multicomponent adsorption. Examples of diffusion studies on the mutual influence of co-adsorbed molecules, as well as on the influence of a carrier gas, are given. Under the conditions of technical application, the deposition of reaction products as well as changes in the structure of the zeolite crystallites and the whole zeolite granule may significantly affect the transport properties. As an example, the influence of a hydrothermal pretreatment on zeolite NaCaA, and the influence of a hydrocarbon atmosphere at elevated temperatures on zeolites NaCaA and HZSM-5 are studied. In all cases considered, changes in the overall transport properties may be correlated with corresponding changes in the elementary processes of molecular migration, caused by structural changes of the adsorbent.

NMR Studies of Single-File Diffusion in Unidimensional Channel Zeolites

Single-file diffusion is the restricted propagation of particles that cannot pass each other. The occurrence of this phenomenon should be reflected by a change in the time dependence of the mean particle displacement in comparison with ordinary diffusion. Although this process is considered to be the rate-controlling mechanism in a large variety of processes, so far no direct evidence of this phenomenon has been provided. Diffusion measurements made with pulsed field gradient nuclear magnetic resonance (NMR) in unidimensional pore systems (zeolites AlPO4-5 and Theta-1) reflect the expected time dependence of single-file diffusion.

Nuclear magnetic resonance studies on the acidity of zeolites and related catalysts

A review is given on the possibilities of the various nuclear magnetic resonance techniques which have been applied up till now to study quantitatively the acidity of zeolites and related catalysts. From the intensity of the wide line proton magnetic resonance signal of unloaded catalysts, the total concentration of protons can be determined. A loading of the samples with (deuterated) pyridine reduces drastically the mean residence time of the acidic protons at the oxygen atoms of the surface through formation and thermally activated motion of pyridinium ions. From the proton magnetic relaxation of the pyridine molecules and pyridinium ions a microdynamical model for the pyridine loaded H-Y zeolites could be derived. The rate constant for the decomposition of the pyridinium-ion-surface-comp)ex which can be determined in this way is used as a measure for the strength of acidity of the OH groups involved. Through magic angle spinning of thermally activated samples (contained in sealed glass ampoules to prevent adsorption of water) it became possible for the first time to determine quantitatively the concentration of non-acidic OH groups, of OH groups having different strength of acidity, and of residual ammonium ions. The results are related to the catalytic activity (cumene cracking) of the amorphous silica-aluminas, Hmordenites and zeolites H-Y studied in the present paper. In contrast to Br~nsted acidity, the current application of n.m.r, techniques to study Lewis acidity is more complicated because experimental difficulties, mainly due to fast exchange of adsorbed bases between different kinds of adsorption sites, arise.

Single-file diffusion and reaction in zeolites

Mass transfer and chemical reaction in channels in which the individual molecules cannot pass each other (single-file systems) are studied by Monte Carlo simulations. Applying a simple jump model for the elementary steps of diffusion, macroscopically observable phenomena like molecular adsorption and desorption, tracer exchange, and counterdiffusion are considered. In the case of chemical reaction, the simulation results are used for a generalization of the Thiele concept to single-file systems.

1H MAS NMR studies on the acidity of zeolites

Proton magic-angle-spinning nuclear magnetic resonance (‘H MAS NMR) spectra contain quantitative information about Bransted acidity and structure defects in z&&es. The strength of acidity of bridging OH groups increases with the Si/Al ratio from 1.4 to 7 but remains constant above Si/Al ~10. Two signals of acidic hydroxyl protons observed in zeolites HY are correlated with the so-called high- and low-frequency band in infrared spectroscopy.

Surface phenomena investigated by nuclear magnetic resonance

Abstract This paper reviews recent achievements in the application of Nuclear Magnetic Resonance to surface phenomena, especially to molecules adsorbed on surfaces of porous crystals. Basic principles of Nuclear Magnetic Resonance are treated only as far as it is necessary to understand potentialities of this method in a study of absolute number (section 2.1), electronic environment (section 2.2), arrangement (section 2.3), and of thermal motion (section 2.4) of nuclei which are part of the absorbate or adsorbent. The structure of an important group of porous crystals, called zeolites of faujasite type, is briefly discussed (section 3.1) and recent results concerning the state of water (section 3.2) and of simple cyclic hydrocarbons (section 3.3) adsorbed in these crystals are reviewed.

Magic-angle spinning nuclear magnetic resonance studies of water molecules adsorbed on Brønsted- and Lewis-acid sites in zeolites and amorphous silica–aluminas

On weakly rehydrated dealuminated zeolites and amorphous silica–aluminas the presence of different types of acidic centres can be ascertained by proton magic-angle spinning nuclear magnetic resonance (1H MAS NMR) measurements. A 1H NMR line at ca. 6.5 ppm is caused by water adsorption on Lewis-acid sites. The shift of the 1H NMR line of Bronsted-acid sites (bridging OH groups) to lower field for hydrated samples can be interpreted quantitatively by a fast proton exchange between water molecules, bridging OH groups and hydroxonium ions.

Microdynamics of methane, ethane and propane in ZSM-5 type zeolites

The n.m.r. pulsed field-gradient technique has been used to study systematically the intracrystalline self-diffusion of methane, ethane and propane in ZSM-5. In conjunction with the information obtained from nuclear magnetic relaxation studies the elementary steps of diffusion are found to be activated jumps between the channel intersections. Only for sorbate concentrations > 2.5 molecules per intersection is a decrease in the jump lengths observed. The results are compared with alkane self-diffusion measurements in zeolites A and X, as well as with the self-diffusion of water in ZSM-5 and zeolites A and X.

N.m.r. studies of aluminium in zeolites

27AI nuclear magnetic resonance (n.m.r.) techniques were used to determine the number of AI atoms in lattice positions and in extra-lattice positions of decationated zeolite Y. A sample activated under deep bed conditions at 400°C exhibits 50% of the total amount of aluminium in extra-lattice positions. One part of the extra-lattice aluminium in the rehydrated samples could be shown to exist in the form of mobile complexes. Further results are that (i) threefold oxygen coordinated lattice aluminium and (ii) OH nests could not be found in the course of the usual processes of dehydroxylation and dealumination. A comparison of the 27A1 n.m.r, shift as determined by magic angle spinning (MAS) of the samples for various cation-exchanged zeolites A, X and Y showed that the resonance line in zeolite A is shifted by 3.5+-1.0 ppm to higher field with respect to zeolite Y. The results are not inconsistent with Loewensteins rule applied to zeolite A, in contrast to interpretations of similar 29Si n.m.r, results published in the literature.

Magic-angle-spinning NMR studies of acid sites in zeolite H-ZSM-5

1H, 11C, 17Al, and 29Si magic-angle-spinning (MAS) NMR was used to elucidate the nature of the catalytic activity of zeolite H-ZSM-5. 1H MAS NMR of sealed samples after mild hydrothermal dealumination shows that the enhanced activity for n-hexane cracking is not due to an enhanced Bronsted acidity. The concentrations of the various OH groups and aluminous species suggest that the reason for the enhanced catalytic activity is the interaction of the n-hexane molecule with a bridging hydroxyl group and with extra-framework aluminium species. Loading the samples with HCOOH or HCl shows that those extra-framework aluminium species, which give rise to the enhanced activity, cannot be easily removed from their positions, and are therefore immobilized by the zeolitic framework.